How to pause a frame in unity , that works even in webgl ?

As i tried to use second camera to pause the frame(by rendering only UI elements before new game objects were loaded into the scene and then enable main camera to render everything) , which worked as intended in unity but there was a glitch in webgl before the first camera starts rendering .

 IEnumerator FreezeCam()
     {
 
 
         //saving previous culling mask and clearing flags
         Previous_mask = Camera.main.cullingMask;
 
         Previous_clear_flags = Camera.main.clearFlags;
 
  
         Camera.main.clearFlags = CameraClearFlags.Nothing;
         Camera.main.cullingMask = 0;
 
         yield return null;
     }
 
  IEnumerator Play_cam()
     {
         //assigning previous culling mask and clearing flags
         
         Camera.main.clearFlags =Previous_clear_flags;
         Camera.main.cullingMask = Previous_mask;

         yield return null;
     }
  public void secondary_camera_setting()
     {
         //Camera.main.enabled = false;
         
         if (secondary_Cam != null)//enabling secondary camera not to render anything
         {
             //to render only UI using culling mask
           secondary_Cam.enabled = true;
             secondary_Cam.cullingMask = 1 << LayerMask.NameToLayer("UI");
 
            
         }
     }
 
    IEnumerator LoadGameObjects()
     {
         ..................
         ..................;
 
         StartCoroutine(Play_cam());
     }
 
     void Start()
     {
     StartCoroutine(FreezeCam());
 
 
     secondary_camera_setting();
 
 
     UnityMainThreadDispatcher.Instance().Enqueue(LoadGameObjects());
 
     
     }